This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Recently, energy harvesting through sustainable approaches has become of interest not only to address the global energy crises but also to provide power for micro-scale electronics and sensors in emerging applications such as wearable and implantable devices. An assortment of technologies has been developed to transform environmental energy into electrical power via a variety of mechanisms, including electromagnetic, electrostatic, piezoelectric, and recently, triboelectric processes. Triboelectric nanogenerators (TENG) are highly capable of efficiently harvesting ubiquitous mechanical energy, hinged on principles of contact triboelectrification and electrostatic induction, and have received considerable attention in recent years. See Wang, Zhonglin, Triboelectric nanogenerators as new energy technology and self-powered sensors—Principles, problems and perspectives. Faraday Discuss, 2014, 176, 447-458. Ongoing efforts are primarily focused on augmenting power generation by increasing triboelectrification surface area, engineering the physical/chemical properties of contacting surfaces and implementing practical applications. Most of the demonstrated TENGs were built based on synthetic polymers for the ease and cost of manufacturability. However, TENGs utilizing naturally abundant biological materials has received considerably less attention. Obstacles concerning practical, eco-friendly utilization of TENGs such as the intricate fabrication and expensive machinery continue to prevail.
Lignin, despite being the second most abundant biopolymer on earth, has few practical applications and a small market value starting from around $300 per ton. Water insoluble Kraft lignin, being the most abundant side product, is produced by the pulp and paper industry at a scale of 50-100 million tons per annum, most of which is nevertheless burned as a cheap fuel, an economically unfeasible solution. Current applications of lignin are scarce, using only approximately 2%-5% of all lignin produced, and primarily utilize lignosulphonate, a chemically modified water-soluble lignin. Lignosulphonate supply is limited, and thus applications of lignin using insoluble Kraft lignin should be conceived. Current utilization of lignin includes binders for animal feed, bricks, ceramics and road dust, in addition to adhesives. This limited employment is mostly due to the undefined, remarkably irregular structure of lignin, which is a highly branched, hydrophobic, three-dimensional biopolymer of p-hydroxyphenyl propanoid units. Still, the presence of highly active functional groups results in lignin being exceptionally accessible for chemical modification or polymerization to create high-value materials e.g. carbon fibers and artificial perfumes. However, many of these applications for Kraft lignin are low yielding or manufactured at a small scale.
Being an antioxidant, naturally degradable, biocompatible, and lacking in cytotoxicity, lignin offers a valuable opportunity as potential constituents in biomedical devices. The vast disparity in structure and surface properties make it finely tunable for controlled degradation which is desirable in implanted applications.
There remains a need to develop new lignin biopolymers and explore the new utilities of such lignin biopolymers.